This photo provided by the New York City Police Department shows Detective Joseph Lemm. Police Commissioner William Bratton said on Monday, Dec. 21, 2015, that Lemm is one of six American troops killed in a suicide attack near Bagram Airfield in Afghanistan.

KABUL, Afghanistan — Fierce battles were under way today between Afghan forces and the Taliban in southern Helmand province where the insurgents have almost completely captured a strategic district as Britain deployed military advisers to the restive area.

The development came a day after a Taliban suicide bomber killed six U.S. troops near a Kabul base — the deadliest attack on Americans in the country since August.

A British Ministry of Defense statement late Monday said that “a small number of U.K. personnel” have been deployed to Helmand “in an advisory role.” The U.K. has 450 troops in Afghanistan as part of NATO’s training mission.

The Afghans were also sending reinforcements to Helmand today, officials said. Afghanistan’s Chief Executive Abdullah Abdullah said the security of Helmand was a “top priority,” according to his deputy spokesman Javid Faisal.

The Taliban have been getting closer to taking full control of the Sangin district for days, with most government buildings now in Taliban hands.

An official in Helmand said only an army base in the district remained in government hands, and it was surrounded by Taliban fighters.

Helmand is important to the Taliban. The lush southern province is home to endless poppy fields and the source of much of the world’s opium, which helps fund the insurgency.

The Taliban claimed responsibility for the Monday suicide attack on U.S. troops that killed six. Two U.S. troops and an Afghan were also wounded when the bomber rammed an explosives-laden motorcycle into a joint NATO-Afghan patrol.

The soldiers were targeted as they moved through a village near Bagram Airfield, the largest U.S. military facility in Afghanistan, NATO and Afghan officials said.

In New York, Police Commissioner William Bratton said Monday that a New York City police detective, Joseph Lemm, was one of the six Americans killed in the attack.

Lemm was a 15-year-old veteran of the New York Police Department and worked in the Bronx Warrant Squad. Bratton says Lemm served in the U.S. National Guard and, while a member of the police force, he had been deployed twice to Afghanistan and once to Iraq. He leaves behind a wife and three children.

Secretary of Defense Ash Carter in statement called the attack “a painful reminder of the dangers our troops face every day in Afghanistan.”